1. Field of the Invention
The present invention relates generally to marine propulsion systems and more specifically to marine waterjet propulsion systems having two or more waterjets.
2. Discussion of the Related Art
Waterjet propulsion whereby a stream of water is hydraulically pumped from a nozzle driven by a pump is becoming increasingly more popular with marine craft manufacturers and their customers. Typically a water inlet port is located upstream of the pump, the inlet port being positioned beneath the water line. The water enters through the inlet port and is directed towards the pump in the inlet duct. The water is then accelerated by the pump out the exhaust outlet port or nozzle thus propelling the watercraft through the water. In addition, there are waterjet propulsion systems that utilize two or more waterjets. For example, U.S. Pat. No. 6,626,713 discloses a water intake and transmission system for simultaneously driving dual waterjet units of a marine vehicle.
A conventional single waterjet system is designed to be able to produce the thrust required to overcome the boat drag at the maximum boat design speed. However, at lower speeds, due to reduced pump power and/or increased boat loads, the propulsive efficiency for a single waterjet is reduced and thus not optimally efficient. In addition, dual waterjet systems such as the system set forth in the aforementioned U.S. Pat. No. 6,626,713 patent will exhibit inefficiencies relative to thrust versus drag at various watercraft speeds when the size of the exhaust outlet ports is fixed and mutually engaged. In general, single or dual waterjet systems with a given cross-sectional diameter or a set of combined diameters are less efficient at low speeds and high drag conditions, the pump being designed to reach its maximum efficiency at a certain watercraft velocity drag ratio.
In light of numerous environmental and cost concerns, efficiency is a major concern with respect to propelling marine vehicles. As well, speed is an important consideration, especially for marine racing vehicles and marine vehicles that are used in pursuit. In addition the dual operation of both waterjets provides more power with a lesser horsepower engine realizing weight and cost reductions and further efficiency gains.
Some efforts have been made to provide multiple waterjet propelled vehicles that are optimized for speed and efficiency, but there is no known history of a marine splitter gearbox which has a single power input and two or more waterjet outputs wherein the waterjets may be independently engaged and may differ in their outlet ports diameters or total effective diameter.
Because the current marine waterjet propulsion systems fail to provide a solution to the problems of lower efficiency and the resulting higher fuel and pollution costs, what is needed then is a waterjet propulsion device for a marine vehicle that is more efficient—at both high and low speeds.